The invention relates to apparatuses for measuring the oxygen content of a gaseous medium, the apparatuses being of the type comprising:
a sensor capable of delivering a voltage representative of the ratio between a reference oxygen pressure and the oxygen pressure in a volume of the sensor which communicates with the gaseous medium via a porous wall; and PA1 monitoring and control means enabling a pumping current to be delivered to cause oxygen to migrate away from or into said volume. PA1 firstly takes advantage of an analog portion that delivers an amplified signal on the basis of the sensor voltage V.sub.s, which signal fortunately has a derivative component of accuracy that is not degraded by analog-to-digital conversion situated downstream from the preparation of said derivative component; and PA1 also makes it possible to produce the current I.sub.p by time modulation in the digital controller, which modulation, in association with a lowpass filter and a current controller governed by the filtered command delivers a pumping current I.sub.p that is variable continuously, progressively, and without interruptions, under the control of the digital controller. PA1 the pumping current is servo-controlled to the voltage applied to the sensitive cell, while verifying that the voltage across the pumping cell remains below the ceiling; and PA1 the pumping current is again governed so that it is compatible with the ceiling insofar as the ceiling is exceeded during the servo-control of the voltage V.sub.s applied to the sensitive cell. PA1 estimating the temperature of the probe, in order to determine the instant at which it becomes usable; PA1 diagnosing aging; and PA1 operating in degraded mode, with servo-control being applied to the voltage V.sub.p of the pumping cell instead of to the voltage V.sub.s.
Numerous apparatuses of that type are already known, such as those described in EP-A-0 507 149 and U.S. Pat. No. 4,932,238. The sensor has at least one sensitive element constituted by a solid electrolyte plate of a type that allows oxygen ions to migrate, the plate being placed between two porous electrodes.
Such a sensor can be the subject of numerous embodiments. FIG. 1 is a diagram of a sensor that can be considered as having two cells. A first cell referred to as a "pumping" cell 10.sub.p is sandwiched between two electrodes 12 and 14. The pumping cell 10.sub.p is fixed to a second cell 10.sub.s, referred to as the "sensitive" cell, via a porous intermediate sheet so as to define a volume 18. Oxygen in the gaseous medium tends to penetrate into the volume 18 so as to bring the oxygen partial pressures into equilibrium. The passage of a current I.sub.p through the pumping cell tends to cause the oxygen contained in the volume to migrate, and thus to maintain the partial pressure therein at a determined value. A final plate 20 can be placed in contact with the sensitive element 10.sub.s and can be made of the same material so as to deliver a constant reference pressure; its usefulness appears below.
Between the electrodes on either side of the sensitive element 10.sub.s there thus appears a measurement voltage V.sub.s representative of the ratio between the partial pressures of oxygen in the volume 18 and in the plate 20 in contact with the cell 10.sub.s. Appropriate solid electrodes, and in particular of doped zirconium oxide or zirconia, have characteristics such that the voltage V.sub.s varies in substantially logarithmic manner with oxygen partial pressure in the volume 18. Conventionally, the current I.sub.p is controlled so as to maintain V.sub.s at a constant value, in which case I.sub.p is representative of the oxygen partial pressure in the gaseous medium. A heating resistor 21 serves to raise the cells to a suitable temperature.
In another embodiment, that can be described as a single cell embodiment and as shown in FIG. 2, the volume 18 is defined solely by the porous intermediate sheet 16 and by the cell 10.sub.p. The reference oxygen partial pressure is then that of atmospheric air, which is in contact with the cell 10.sub.p. Under such circumstances, variation of I.sub.p as a function of V.sub.p for different oxygen partial pressures has the general appearance shown in FIG. 3. Insofar as it is desired to remain at all times within a rectilinear portion of the characteristic, the value to which V.sub.p is servo-controlled must depend to some extent on the partial pressure of oxygen in the gas and on the impedance of the cell.
A major application of the invention lies in determining the air/fuel ratio admitted into an internal combustion engine on the basis of the composition of the exhaust gas, and more particularly on the basis of the partial pressure of the residual oxygen in the exhaust gas.
Until now, use has been made above all of sensors of the kind shown in FIG. 1. Often the monitoring and control means are constituted by an analog loop for servo-controlling V.sub.s to a constant value, associated with a microcontroller which deduces the oxygen partial pressure and the instantaneous richness of the mixture from the value of the current I.sub.p. That solution suffers from drawbacks. The accuracy with which richness is measured is limited by the accuracy with which I.sub.p is measured. Embodiment in hard-wired form reduces possibilities of adjustment and matching.